The invention relates to a switching arrangement for increasing stability in a polyphase power system fed by a generator.
A switching arrangement of the type to which the present invention relates is known, e.g., from the 1973 article by O'Kelly, D. and G. Musgrave, entitled "Improvement of power-system transient stability by phase-shift insertion," Proc. IEE [(Lon.)], v. 120 no. 2 (Feb.), pp. 247-252.
The known switching arrangement is suitable for increasing transient stability in a polyphase power system. "Transient stability" in this context is stability with respect to so-called "null mode excitation". The known switching arrangement is also often referred to as "slip stabilizing".
In addition to the null excitation mode, polyphase power systems, particularly those which are series compensated with capacitors and have an eigenfrequency near an eigenfrequency of the mechanical system comprising the generator and its driving turbine, also have further excitation modes known as modes 1, 2, 3, etc. These excitation modes have frequencies which are between 4 and 40 Hz below the power network frequency, and may also be referred to as "subsynchronous oscillations" or "subsynchronous resonances". In a mechanical system comprised of a generator and a turbine, subsynchronous oscillations may be, e.g., torsional oscillations of the individual rotating masses, particularly in a case where the turbine is comprised of separate high-pressure, medium-pressure, and low-pressure sections. In a number of instances in the past, such subsynchronous oscillations have led to equipment damage.
The known switching arrangement does not enable subsynchronous oscillations to be damped.